Phosphate coating of metallic articles



Patented Au 23, 1949 PHOSPHATE COATING F METALLIC ARTICLES Lloyd 0. Gilbert, Davenport, Iowa No Drawing. Application September 14, 1945, Serial No. 616,435

1 Claim.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to phosphate coating of metallic articles and more particularly to a method of producing greatly improved phosphate coatings. This invention has application in the field in which rustproof coatings of metallic phosphates are formed upon articles having a surface of iron, zinc, cadmium, magnesium. aluminum, and alloys thereof by immersing the article in an acidic aqueous solution comprising metallic phosphates, such as ferrous phosphates and metallic phosphates selected from the group consisting of zinc phosphates, manganese and copper phosphates.

I have found that improved coatings are produced as a result of adding to the phosphate coating bath a compound of boron, which will liberate boric acid in the bath at a relatively slow rate. I prefer to use a metallic fiuoborate, boron phosphate or fluoboric acid.

It is a characteristic of the boron compounds which I desire to use that they hydrolyze relatively slowly in phosphate coating baths to yield boric acid. As distinguished from this, the borates hydrolyze Very rapidly to give boric acid, and hence are undesirable for employment in accordance with the invention.

It is known that additions of boric acid to phosphate coating baths result in improvement in the finishes produced upon metallic articles treated in the bath. However, the addition of boric acid is accompanied by many undesirable results. .The. baths are operated at elevated temperatures slightly below the boiling point, and, at these temperatures, the boric acid is rapidly lost from the bath by volatilization or steam distillation; hence its beneficial effects are short lived. Because of the rapid loss of boric acid, frequent replenishment is necessary.

The invention will be more particularly described in reference to the coating of ferrous metals utilizing a bath containing ferrous phosphates, zinc phosphates and a-nitrate. It Will be understood, however, that this is by way of example only and is'not to be construed as limiting the invention.

The following table gives the characteristics of crystalline structure. proved corrosion resistance as compared to the a typical and satisfactory phosphate coating bath:

The free acidity is measured in conventional manner by titrating 10 cc. sample of the bath with 0.1 N sodium hydroxide to the methyl orange end point; the number of cos. of sodium hydroxide required is the point value of the free acidity. The total acidity is determined by titrating a 10 cc. sample of the bath with 0.1 N sodium hydroxide using phenolphthalein indicator; the number of cos. of sodium hydroxide required is the point value of g the total acidity. Iron, zinc and nitrate are determined by conventional quantitative analytical methods.

The bath described immediately above when operated at a temperature of from 205 F. to 210 F. produced a satisfactory coating on a steel article immersed in the solution for a period oi. one-half hour.

To the bath described above. which had a volume of 270 gallons, was added one pint of an aqueous-solution of zinc fluoborate having the following analysis:

- Percent Zn(BF4)2 .4. 47.55 Free HBF4 1 0.52 Free H3303 4.11

After the addition of the solution of zinc fluoborate to the bath, a steel article was treated therein for a period of one-half hour. 'The article was found to be coated with a much darker almost black film exhibiting a. uniformly fine Such article showed imarticle similarly prepared in a bath containing no zinc fiuoborate.

It has been found that quantities of fluoborate greater than one pint of zinc fluoborate in 2'70 gallons of phosphating solution do not materially improve the quality of the coating produced. However, the efiect of the addition extends over a longer period of time. For example, the effects of one pint of fluoborate may last for three to four days and the eiiects oi the addition of two pints may last from six to eight days. Even very large additions oi zinc fluorborate up to as much as tour or five gallons per 2'10 gallons 0! phosphate bath do not have any deleterious action. Buch The formation of the zincfluoride and the hydrofluoric acid is believed to add to the effectiveness of the bath by reducing the temperature at which the bath may be operated for the production of a superior coating.

' Other metallic fluoborates may be employed in a manner siinilar to that described for the employment of zinc fluoborate. Manganese fluoborate [Mn(BF4)sl may be advantageously used in phosphating baths containing a mixture oi ferrous and manganese phosphates. Where copper is used in the coating bath, additions of copper fiuoborate have been found to produce better coatings. Fluoborio acid ma be employed it desired. Boron phosphate [BPOd also may be used to advantage in any of the phosphating baths. It has been found that additions of from one-hali' pound to three pounds of boron phosphate to 240 gallons 01' a zinc-ferrous phosphatin bath as set forth above will condition the bath with,a

2 half pound to about three .30 Number production of superior coatings for a period of 8 from four days to three weeks.

4 It is believed that boron phosphate hydrolyles to yield boric acid and phosphoric acid in accordance with the following:

consisting 0t zinc, manganese and copper, and

boron phosphate in the proportion of about onepounds per about 240 gallons oi said solution.

LLOYD o. GILBERT.

REFERENCES crrEn The following references-'are. of record in the flie of this patent:

UNITED STATES PATENTS Name Date 1,610,362 Coslett Dec, 14, 1926 1,946,647 Taylor et al Feb. 13, 1934 2,310,381 Zimmer et a1 Feb. 9, 1943 2,312,855 Thompson Mar. 2, 1943 

